1. Field of the Invention
This invention relates to multibeam light sources for use in information recording systems such as a laser printer, digital duplication machine, facsimile apparatus and other similar systems, and more particularly to such multibeam light sources provided with semiconductor laser arrays.
2. Discussion of the Background
In many recent developments of laser printers, digital duplication machines and facsimile apparatuses, attention has been focused on the increase in speed and density of information recording. In order to achieve such improvement, a multibeam type light source has been developed which is provided with a plurality of laser beams simultaneously scanned over a recording substrate.
As an exemplary light source incorporated into a conventional multibeam light source, a semiconductor laser array is formed, including a plurality of light emitting points aligned on a single substrate, as disclosed in, for example, Japanese Laid-Open Publications Nos. 66-42248, 9-26550, 8-136841 and 9-261137.
Japanese Laid-Open Publication No. 8-136841 discloses a multibeam light source having two laser beams, in which the beam path for the first beam is brought to in coincidence with the center of a rotatable member, while the beam path for the second beam is displaced along the rotation of a rotatable member. As a result, the distance between these two beam paths (or the interval of recorded dot density) may suitably be adjusted.
Also, in the multibeam light source (in a laser recording apparatus) disclosed in Japanese Laid-Open Publication No. 9251137, the positions of light beams emitted from the laser array are detected with respect to the primary and secondary scanning directions for the recording apparatus using an index sensor. The index sensor is formed incorporating four constituent sensing portions, each being of rectangular shaped and corresponding to light beam detection (light receiving) area. Based on the result from detection, the laser array is then rotated to suitably adjust the interval, in the secondary scanning direction, of the plurality of light beams emitted from the laser array.
As shown in FIG. 12 of Japanese Laid-Open Publication No. 9251137, this light source includes a laser array 31 provided with four light emitting points E1˜E4 formed on a chip. The chip is, in turn, capsulated in a holder 32 and rotatably incorporated into the light source. In addition, the light source is adjusted such that one of the light emitting points E1 at the end of the array be situated at the center of rotation for the laser array.
Also disclosed are multibeam light sources which are capable of scanning a plurality of laser beams, such as those described in Japanese Laid-Open Publications Nos. 10-39241, 9-251137, 9-1861 and 9-211350.
Japanese Laid-Open Publication No. 10-39241 discloses a multibeam light source capable of adjusting the rotation angle of its laser array depending on the selection of the scanning density. In contrast, a laser array included in the multibeam light source disclosed in the Japanese Laid-Open Publication No. 9-251137 may rotatably adjust the pitch along the secondary scanning direction of the array based on the results from detected beam position with respect to the primary and secondary scanning directions.
In addition, disclosed in the Japanese Laid-Open Publication No. 9-1861 is a multibeam light source with the capability of adjusting the phase difference along the primary scanning direction between a plurality of laser beams. Also disclosed in the Japanese Laid-Open Publication No. 9-211350 is a multibeam light source, in which dot writing positions may suitably be corrected even after transforming the beam pitch.
Several multibeam light sources are thus previously known, being provided with a plurality of light emitting points formed on a chip. However, the reduction of the distance between neighboring emitting points is limited so far to the order of, for example, 100 μm at least, because of appreciable interference effect between laser beams, although the distance may be decreased to equal to, or less than 20 μm, for example with the recent improvements in semiconductor manufacturing technology.
Multibeam light sources provided with a plurality of light emitting points formed with the thus decreased spacing are exemplified by those disclosed in Japanese Laid-Open Publications Nos. 9-251137, 9-211350 and 9-1861.
In these light sources, however, there are found several shortcomings. For example, the multibeam light source in laser recording apparatus disclosed in the Japanese Laid-Open Publication No. 9-251137 has to incorporate the aforementioned complicated sensors and beam detection algorithm, which result in the increase in machine costs.
For previous light sources, in addition, when the deviation of laser beam spots projected onto recording substrates is found to such an extent as to appreciably affect the quality of recorded images, correction for dot writing positions is often necessitated. These examples are found in the multibeam light sources disclosed in the Japanese Laid-Open Publications Nos. 9-211350 and 9-1861.
Furthermore, in the multibeam light source disclosed in Japanese Laid-Open Publication No. 8-136841, a beam path for the first beam is brought to coincide with the center of a rotatable member, while the second beam path is displaced along the rotation of the rotatable member, so as to adjust the distance between these two beam paths. The second beam path is therefore situated further away from the optical axis of the collimator lens than the first beam path.
This gives rise to a difficulty, in that, since the position of the beam waist is different for respective beams, desirable diameter of each beam suitable for image recording can not be achieved.
Further, the multibeam light source disclosed in Japanese Laid-Open Publication No. 9-261137 is adjusted so that the center of rotation for the laser array 31 is situated at the light emitting points E1 at one end of the array, as shown in FIG. 12. Therefore, the emitting point E4, which is situated at the opposing end to the emitting point E1, is far away from the optical axis and may not be able to form a suitable beam shape onto the recording substrate. This may also give rise to a shortcoming, in which high quality of recorded images can not be achieved.
In the multibeam light sources, therefore, further improvements are desirable to implement laser beam scanning capable of increasing speed and density of the information recording.